1. Technical Field
The present invention relates to a data transmission control system and, particularly, to a data transmission control system in a set top box(hereinafter, referred to STB) for matching timings between transmission data and synchronous signal of the data exactly, in case that transmission paths of the transmission data and the synchronous signal in the STB are different.
2. Background
Generally, a data transmission control system in a conventional STB has a demultiplexer DEMUX 10 for receiving compressed video/audio bitstreams from a video server and demultiplexing the bitstreams, a microprocessor 20 for synchronizing a presentation signal with a system clock, and a video decoder 30 and an audio decoder 40 for encoding the video/audio bitstreams and providing the bitstreams to a video output device and an audio output device, shown in FIG. 1.
The DEMUX 10 demultiplexes the compressed video/audio bitstreams transmitted from the video server using MPEG-2(Moving Picture Experts Group-2) to a Presentation Time Stamps PTS and a video bitstream ES(Elementary Stream) and an audio bitstream ES. The audio ES is transmitted through a transmission path connected to the audio decoder 40. The video ES is transmitted through a transmission path connected to the video decoder 30. Here, the PTS is transmitted to the video decoder 30 via the microprocessor 20 in order to be synchronized with the video ES. Accordingly, transmission time of the PTS affects the synchronization of the video signal.
The video decoder 30 allocates the PTS on the basis of a sequence header inside of the video ES. Referring to FIG. 2 the video ES and the PTS are synchronized with PTS1 and ES1, PTS2 and ES2, and PTS3 and ES3, respectively when the video signal is normally synchronized. Thereby, the video decoder 30 may decode video data exactly.
Further information can be found in copending U.S. Pat. No. 5,559,999, entitled "MPEG decoding system including tag list for associating presentation time stamps with encoded data units", which is incorporated by reference herein.
However, the PTS may be not synchronized with the video ES in case that the PTS is applied to the video decoder 30 upon being delayed from the microprocessor 20, referring to FIG. 3. Here, the video decoder 30 recognizes that the first PTS 1 is a synchronization signal of the ES1 correctly. But, the ES 2 signal has already applied to the video decoder 30 when the PTS 2 signal is applied to the video decoder 30. Accordingly, the video decoder 30 misses the ES2 sequence header and recognizes that the PTS2 signal is the synchronization signal of the ES3, and the synchronization between the PTS signal and the ES signal is therefore asynchronized.
Consequently, there are problems that the presentation screen is distorted or trembled and also lip sink is not matched with audio when the PTS and the video ES are asynchronized.